igbvf: remove unused interrupt counter fields from struct igbvf_adapter

Remove `int_counter0` and `int_counter1` from struct igbvf_adapter since
they are only incremented in interrupt handlers igbvf_intr_msix_rx() and
igbvf_msix_other(), but never read or used anywhere in the driver.

Note that igbvf_intr_msix_tx() does not have similar counter increments,
suggesting that these were likely overlooked during development.

Eliminate the fields and their unnecessary accesses in interrupt
handlers.

Tested-by: Kohei Enju <enjuk@amazon.com>
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>